Rapid movement is a necessity, particularly in the case of the hydraulic controls of high-power circuit-breakers, where the release of the circuit-breaker must be effected very rapidly after the occurrence of a fault, for example, a short-circuit.
Until the present time, with circuit-breakers for power transmission lines, an acceptable initiation period was of the order of 10 milliseconds (a half-cycle in AC at 50 cycles), but it would be desirable, because of the increase in interconnected power and with the purpose of limiting peaks, to achieve reduced shut-off times, preferably of less than 5 ms (a quarter-cycle), and of about 2.5 ms, for example, after the occurrence of a fault.
In various types of hydraulic control for circuit-breakers, the circuit-breaker is kept engaged by oil pressure against elastic release means which are always available (such as mechanical springs, pneumatic springs, hydropneumatic pressure, hydraulic compression, or the like). In order to release the device, in response to a release signal, it is enough for the hydraulic jack keeping the circuit-breaker engaged to be turned to drain. The initial release signal is most frequently an electrical signal controlling a draining electrovalve.
The occurrence of a fault on an electrical power line is detected by the safety devices. Electronic safety devices have very short intrinsic operating times which are measured in microseconds. In order to avoid undesirable release of circuit-breakers, and in particular in order to filter out transient disturbances at high frequency, these safety devices must be given a "thinking time" or delay, which must be at a minimum of the order of 0.5 milliseconds. Therefore, after issue of the electrical release signal, there remains a delay of 2 milliseconds for bringing the contact of the circuit breaker into the cut-off position, if the purpose indicated above has been selected.
This delay may be divided into four portions:
a. emission of the hydraulic low-output drain signal; PA1 b. hydraulic transmission of this order to a high-output valve; PA1 c. decompression of the jack; PA1 d. draining and lowering of the jack.
In order to obtain very brief response times of the electrovalve providing the hydraulic drain signal, the moving parts of this device must be of low inertia, and have a small path of travel; the device must therefore be of small dimensions, which leads to a low-output electrovalve. Until the present time, in hydraulic controls, particularly for circuit-breakers, this low-output drain electrovalve was used as a pilot valve for a valve with a greater output, which was controlled hydraulically, and which set the high-pressure vessel to drain.
It became apparent that, in existing hydraulic controls functioning at oil pressures of the order of 300 to 500 kg/cm.sup.2, the problem of rapid draining of a high-pressure vessel resided not only in the high-output evacuation of a certain volume of oil, but firstly in the decompression time of this oil. In fact, at these pressures, and when time lapses of the order of milliseconds are envisaged, the compressibility of the oil is a disadvantageous delaying factor of some importance.
Thus, as will be seen in more detail below, the compressibility at 300 kg/cm.sup.2 of the oil contained in a 1 liter jack represents a volume of 15 cm.sup.3. This is to say that this volume of 15 cm.sup.3 must first of all be evacuated, at the beginning of draining of the jack, before any mechanical action controlled by the jack may really commence. The previous evacuation of this 15 cm.sup.3 by a duct with a diameter of 20 mm would already require a delay of about 0.5 milliseconds, which is of course increased by the existence of "dead spaces" in the hydraulic controls. It is evident that lost time of 0.5 milliseconds must be taken into consideration when an overall time lapse of about 2 milliseconds is being aimed at.